1. Field of the Invention
This invention relates to radio frequency oscillators, and particularly to a frequency modulated oscillator having improved linearity over an extended modulation range.
2. General Description of the Prior Art
Frequency modulated transmitters for telemetry and television applications at S-band and higher frequencies require the use of an oscillator that can be modulated linearly in frequency over a wide range at high rates. Although the instantaneous frequency of the oscillator is varied at the modulation rate, the average frequency must be kept constant to very close tolerances. For speech and music modulation in which the average value of the modulating signal is zero, a closed-loop automatic-frequency-control (AFC) system provides an excellent means for stabilizing the frequency.
In systems where the information to be transmitted is in a well-defined, synchronous form, the frequency error can be corrected at specified times during transmission. For example, in a frequency-modulated television transmitter, the frequency can be stabilized during the horizontal blanking interval of the television signal. However, if the information to be transmitted is not conveniently synchronized, or if the system is to be used to transmit information in more than one format, the closed-loop arrangement may be impractical. Linearizing circuits have been used with varactor modulated oscillators in an effort to improve performance. These consist of several diodes and biasing voltages which provide a piecewise linear approximation of the desired correction curve. However, the circuits are highly temperature dependent and do not respond to high modulation frequencies.
For many telemetry applications, a system is desired which will generate and transmit information in any format, analog or digital, with a high degree of frequency stability, employing a frequency deviation as high as one percent of the microwave carrier frequency. Ideally, the oscillator would have a broadband modulation response extending from D.C. to several megahertz.
It is, accordingly, the object of this invention to provide an improved frequency modulated oscillator having these characteristics.